Among printing systems to print images including characters on a print medium such as paper and thin plastic sheets (e.g., OHP), there is a liquid printing system that performs contact or noncontact printing on a print surface of the print medium by applying a liquid ink to the print medium.
An ink jet printing system in particular has many advantages, such as a high speed printing, an ease with which a color printing can be made, a capability to print on print mediums such as paper and cloth, small noise, and a capability to produce high quality printed images.
As an ink supply source for a variety of printing apparatus including the ink jet printing apparatus, a cartridge type ink tank is employed. Most of the ink cartridges employed are detachable from the printing apparatus.
The printing apparatus has a mechanism to detect ink remaining in the cartridge to prevent an inadvertent interruption of the printing operation that would otherwise occur when the cartridge runs out of ink. The mechanisms for detecting the presence of the ink remaining in the cartridge include one using a means of detecting a conduction state between electrodes in the cartridge and one using a means for optically detecting ink.
Particularly, the means for optically detecting the presence or absence of ink is simple in construction and does not require a large device and therefore has found many applications.
As an example of such a mechanism for optically detecting a remaining ink, a mechanism using a prism is proposed in U.S. Pat. Nos. 5,616,929 and 6,361,136. The remaining ink detection mechanism using a prism has a light emitting portion and a light receiving portion on the printing apparatus side and, on the ink cartridge side, a prism reflection surface made of a light transmitting material such as polypropylene. When the ink in the cartridge runs out and an interface of the prism changes from ink to air, light from the light emitting portion on the printing apparatus side is totally reflected by the prism reflection surface and detected by the light receiving portion on the printing apparatus side. Based on a change in optical reflection intensity, whether or not there is an ink in the cartridge is determined.
The mechanism that detects the presence or absence of ink in the ink cartridge according to a change in optical reflection intensity, however, has a possibility of the following problem.
When an output of the light emitting portion is increased to improve an accuracy of detecting the remaining ink, unwanted, irregularly reflected rays and scattered rays are produced from the light emitted by the light emitting portion in addition to the reflected light that needs to be detected. These unwanted rays may reach the light receiving portion at a relatively large intensity, making the precise ink detection difficult. Setting high an output threshold for detecting the presence or absence of ink to cope with these irregularly reflected rays and scattered rays of relatively large intensity, however, may pose another problem of a degraded precision of the remaining ink detection and a detection delay.